class Solution {
public:
    int getMissingNumber(vector<int>& nums) {
        unordered_set<int> S;
        for (int i = 0; i <= nums.size(); i++) {
            S.insert(i);
        }
        for (auto x : nums) {
            S.erase(x);
        }

        return *S.begin();
    }
};
